1. Field of the Invention
The present invention relates to an image pickup apparatus which detects a contrast focal point and displays an image based on image signals received from an image pickup device.
2. Description of the Related Art
In electronic image pickup apparatuses, images outputted from image pickup devices at predetermined frame rates have been often used for live views through EVFs or back liquid crystal display panels, or for picking up video images. In image pickup apparatuses such as recent digital cameras, models which can pick up live view images having about the number of pixels of SVGA and video images having the number of pixels of HD video images have become popular. For example, recording and displaying frame rates are standardized for HD video images, and an image pickup frame rate for an HD image pickup mode is 60 fps, for example.
Also, contrast AF for carrying out focal point detection based on an image outputted from an image pickup device is widely used in electronic image pickup apparatuses because advantageously, an additional focal point detecting device and the like is not needed.
Because the contrast AF searches for a peak position of contrast components (e.g., high frequency components) in image signals while moving a focus position of a lens by a hill-climbing scheme, multiple frames of image signals are needed to achieve focus.
In the contrast AF, although it is necessary to extract reliable contrast components from an image signal, if a subject is under insufficient light, a ratio of noise components to signal components becomes high. Thus, for example, Japanese Patent Application Laid-Open Publication No. 2004-289870 describes a technique for, if a subject is under low light, decreasing a frame rate to obtain an image signal with a smaller number of noises and performing contrast AF or image display.
On the other hand, to increase AF speed, a frame rate of an image obtained for focal point detection may be increased. For example, for CMOS image pickup devices, in order to increase a frame rate of an image, a technique is known which reduces the number of pixels outputted from an image pickup device by pixel thin-out read-out, pixel summation read-out or the like.
An image pickup device incorporating a pixel portion and an analog memory in which an analog image signal read out from the pixel portion is stored has been proposed. For example, there is a technique described in Japanese Patent Application Laid-Open Publication No. 2010-252118.
In contrast, it may also be contemplated that a high-resolution image is outputted from an image pickup device at a high frame rate and then processed by an ASIC or the like. However, if establishment of both contrast AF and pickup of HD video images is attempted, as described above, because an image pickup frame rate of an HD image pickup mode is limited to 60 fps, the AF speed of the contrast AF is also limited. Thus, in order to further establish high-speed contrast AF at the same time, it may also be contemplated that an image pickup frame rate is set to integer multiples of 60 fps, for example, 120 fps to add frames for an image to be stored/displayed.
Also, as a frame rate increases, a charge storage time for an image is shortened. Thus, in addition, it is desirable to be able to improve reliability of contrast AF if a subject is under low light.